Integrated circuit packaging

ABSTRACT

An integrated circuit package and methods for packaging an integrated circuit. In one example, a method for packaging an integrated circuit includes connecting input/output pads of a first die to terminals of a lead frame via palladium coated copper wires. An oxygen plasma is applied to the first die and the palladium coated copper wires. The first die and the palladium coated copper wires are encapsulated in a mold compound after application of the oxygen plasma.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional Patent Application No. 62/611,818, filed Dec. 29, 2017, entitled “Integrated Circuit Packaging,” which is hereby incorporated herein by reference in its entirety.

BACKGROUND

Isolation is used in electrical circuits to prevent the flow of direct currents and undesirable alternating currents between two parts of a system. While preventing flow of undesirable currents between parts of a system, isolation may allow signal transfer between the isolated parts of the system. Isolation may be provided by integrated circuits referred to as isolators. Isolators may be included in electronic systems for safety reasons and/or to protect electronic components of the systems. For example, where two systems need to communicate, but the systems have grounds that may be at different potentials, communication may be through an isolator that is tied to the grounds of both systems but allows no current flow between the grounds. Various types of isolators may include optical coupling, capacitive coupling, inductive coupling, or other types of coupling to isolate systems while allowing communication between the systems.

Like other integrated circuits, isolators are provided in packages that typically include at least one die and at least one substrate or lead frame to which the die is affixed by an attachment medium such as solder or epoxy. The substrate facilitates electrical attachment of the die to other circuits. The various components of the integrated circuit package are generally encased in a protective mold compound, such as epoxy.

SUMMARY

An integrated circuit package and methods for packaging an integrated circuit are disclosed herein. In one example, a method for packaging an integrated circuit includes connecting input/output pads of a first integrated circuit die to terminals of a lead frame via palladium coated copper (PCC) wires. An oxygen plasma is applied to the first integrated circuit die and the PCC wires. The first integrated circuit die and the PCC wires are encapsulated in a mold compound after application of the oxygen plasma.

In another example, an integrated circuit package includes a first die, a second die, a lead frame, palladium coated copper bond wires, gold bond wires, and encapsulation material. The PCC bond wires conductively couple the first die and the second die to the lead frame. The gold bond wires conductively couple the first die to the second die. The encapsulation material encases the first die, the second die, the lead frame, the PCC bond wires, and the gold bond wires.

In a further example, a method for packaging an integrated circuit includes connecting input/output pads of a first die to terminals of a lead frame via PCC wires. The first die is connected to a second die via gold wires. The lead frame, the first die, the second die, the gold wires, and the PCC wires are encapsulated in a mold compound.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of various examples, reference will now be made to the accompanying drawings in which:

FIG. 1 shows an Energy Dispersive Spectroscopy (EDS) spectrum of a palladium coated copper bond wire prior to application of a plasma in accordance with various examples;

FIG. 2 shows an EDS spectrum of a palladium coated copper bond wire after application of argon plasma in accordance with various examples;

FIG. 3 shows a flow diagram for a method for packaging an integrated circuit in accordance with various examples;

FIG. 4 shows an EDS spectrum of a palladium coated copper bond wire after application of oxygen plasma in accordance with various examples; and

FIG. 5 shows a diagram of an isolator circuit assembly connected with palladium coated copper bond wires in accordance with various examples.

DETAILED DESCRIPTION

Certain terms have been used throughout this description and claims to refer to particular system components. As one skilled in the art will appreciate, different parties may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In this disclosure and claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” or “couples” is intended to mean either an indirect or direct wired or wireless connection. Thus, if a first device couples to a second device, that connection may be through a direct connection or through an indirect connection via other devices and connections. The recitation “based on” is intended to mean “based at least in part on.”Therefore, if X is based on Y, X may be a function of Y and any number of other factors. The term “approximately” is intended to mean within 10% of a stated amount. Thus, approximately 100 specifies a range of 90-110.

Cost reduction in packaged integrated circuits, such as packaged isolator integrated circuits, is generally desirable. As the cost of gold bonding wire has risen over time, bonding wire made of copper or other metals has become available as a lower cost alternative to gold. Copper has a number of advantages over gold, but use of copper bond wire presents a number of challenges. For example, copper oxidizes at relatively low temperatures. Copper bond wire may be coated with palladium to inhibit oxidation. Retention of the palladium coating throughout the packaging process is important to maintain the integrity of the bond wire.

At various points in the integrated circuit packaging process, the assembled components may be cleaned to prepare the assembly for further processing. For example, the integrated circuit assembly may be cleaned to facilitate adhesion in an upcoming packaging operation. In some integrated circuit packaging processes, a circuit assembly may be cleaned by application of a plasma. Plasma is an ionized gas that can effectively remove contaminants from the surfaces of a circuit assembly. For example, plasma may be applied to a die to prepare the die for wire bonding. Plasma is formed by radiating radio frequency power, e.g., generating a radio frequency oscillating electric field using capacitive plates or magnetic induction, into a gas.

In packaging of integrated circuits (e.g., isolator integrated circuits), an argon plasma may be applied to the circuit assembly after the die is wire bonded to the package lead frame. The argon plasma cleans the surfaces of the circuit assembly in preparation for encapsulation. If the surfaces of the circuit assembly are not adequately cleaned prior to encapsulation, then friction with the surfaces of the circuit assembly may cause voids in the encapsulation material to form about the circuit assembly that can lead to failure of the packaged integrated circuit. For example, a void may form in an area with high electric field, with the void residing between high and low voltage nodes within the circuit assembly. The void, which can be represented as a region of poor breakdown strength isolation within the packaged device between high and low voltage nodes, can reduce the lifetime of the packaged device. Thus, voids in the encapsulation material encasing an isolator should be prevented to reduce premature failures in the isolator.

While use of argon plasma presents no issues with gold bond wires, argon plasma can cause palladium and copper to sputter off of palladium coated copper (PCC) bond wire. FIG. 1 shows an energy dispersive spectroscopy (EDS) spectrum of a PCC bond wire prior to application of a plasma. The peak 102 represents copper signal intensity and the peak 104 represents palladium signal intensity. The peak 102 is lower than the peak 104 which indicates that the copper signal intensity is lower than the palladium signal intensity in the PCC bond wire prior to application of plasma.

FIG. 2 shows a spectrum of the palladium coated copper bond wire after application of argon plasma. The peak 202 represents copper signal intensity and the peak 204 represents palladium signal intensity. The spectrum indicates that relative amplitude of the copper and palladium signal peaks has changed (as compared to the peaks of FIG. 1). For example, the peak 202 may not be lower than the peak 204. The change in the relative amplitude of the copper and palladium signal peaks is caused by the sputtering of palladium from the PCC bond wire by the argon plasma.

Embodiments of the present disclosure avoid or reduce the loss of palladium from PCC bond wires without increasing the formation of voids in the encapsulation material. The packaging methods disclosed herein include a cleaning process that employs an oxygen plasma, rather than an argon plasma, to clean the circuit assembly after the palladium coated copper bond wires are attached. The oxygen plasma cleans the circuit assembly, and produces a low wetting angle on the surfaces of the circuit assembly in preparation for application of encapsulation material. Because of the low wetting angle produced by the oxygen plasma, voids are averted in the contact area of the circuit assembly and the encapsulation material, which in turn reduces the failure rate of the packaged device. Use of PCC, rather than gold, bond wires substantially reduces the cost of the packaged device.

FIG. 3 shows a flow diagram for a method 300 for packaging an integrated circuit in accordance with various examples. Though depicted sequentially as a matter of convenience, at least some of the actions shown can be performed in a different order and/or performed in parallel. Additionally, some embodiments may perform only some of the actions shown. In some embodiments, the integrated circuit packaged using the method 300 is an isolator.

In block 302, a first die is attached to a package lead frame, and a second circuit die is attached to a package lead frame. In some embodiments one or more circuit dice are attached to a package lead frame in block 302. The one or more die may be attached to the lead frame with an epoxy-based adhesive, solder, or another bonding agent or attachment device. FIG. 5 shows a diagram of an isolator circuit package 500. In FIG. 5, a die 504 is attached to a lead frame 502, and a die 506 is attached to the lead frame 502.

In block 304, an argon plasma is generated and the argon plasma is applied to the package produced in block 302. That is, the argon plasma is applied to the lead frame and the one or more dice affixed to the lead frame to clean the surfaces of the lead frame and the one or more dice affixed to the lead frame in preparation for attachment of bond wires.

In block 306, input/output pads of the one or more dice attached to the lead frame are conductively connected to terminals of the lead frame using PCC bond wire. For example, a first end of a PCC bond wire is attached to an input/output pad of a die and a second end of the PCC bond wire is attached to a terminal of the lead frame. In FIG. 5, a first end of the PCC bond wire 510 is attached to the input/out pad 508 of the die 506 and a second end of the PCC bond wire 510 is connected to the terminal 512 of the lead frame 502. Some terminals (e.g., power terminals) of the lead frame 502 may be connected to a die (504 or 506) by multiple PCC bond wires. The PCC bond wire may be attached to the input/output pads of the one or more dice and to the terminals of the lead frame using welding operations such as ball bonding, stitch bonding, and/or wedge bonding.

In block 308, the first circuit die 504 is conductively connected to the second circuit die 506. In some embodiments, gold bond wire is used to conductively connect the first circuit die 504 to the second circuit die 506. In some embodiments, palladium coated copper bond wire is used to conductively connect the first circuit die 504 to the second circuit die 506. FIG. 5 shows the circuit die 504 connected to the circuit die 506 using gold bond wires 514. Attachment of the bond wires connecting the first circuit die 504 to the second circuit die 506 may be by welding. In some embodiments (e.g., if the dice 504 and 506 form an isolator), a first end of the bond wire 514 is connected to a plate of a capacitor 516 formed on the first circuit die 504 and a second end of the bond wire 514 is connected to a plate of a capacitor 518 formed on the second circuit die 506. Communication between the first circuit die 504 and the second circuit die 506 via the capacitors 516, 518 connected by the bond wire 514 provides galvanic isolation between the first circuit die 504 and the second circuit die 506.

In block 310, an oxygen plasma is generated and the oxygen plasma is applied to the package produced in block 308. That is, the oxygen plasma is applied to the lead frame 502, the one or more dice (e.g., die 504 and die 506) affixed to the lead frame 502, the PCC bond wires 510 that connect the lead frame 502 to the one or more dice, and the bond wires 514 that connect the dice to one another. The oxygen plasma cleans the surfaces of the package in preparation for encapsulation. Palladium is not sputtered from the PCC bond wires 510 by the oxygen plasma. Wetting angles achieved on surfaces of the lead frame 502, the one or more dice (e.g., 504 and 506) affixed to the lead frame 502, the PCC bond wires 510 that connect the lead frame 502 to the one or more dice, and the bond wires 514 that connect the dice to one another after application of the oxygen plasma are comparable to those obtained using argon plasma (e.g., less the 30°).

FIG. 4 shows a spectrum of a PCC bond wire after application of oxygen plasma in accordance with various examples. The peak 402 represents copper signal intensity and the peak 404 represents palladium signal intensity. Like the spectrum for the pre-plasma palladium coated copper bond wire shown in FIG. 1, the peak 402 is lower than the peak 404 which indicates that the copper signal intensity is lower than the palladium signal intensity. Thus, application of the oxygen plasma does not sputter palladium from the PCC bond wire as does the argon plasma, which results in a uniform amount of palladium coating the copper wires.

In block 312, after application of the oxygen plasma, an encapsulation material (e.g., an epoxy mold compound) is molded around the lead frame 502, the one or more dice (e.g., die 504 and die 506) affixed to the lead frame 502, the PCC bond wires 510 that conductively connect the dice to the lead frame, and the bond wires 514 that connect the dice to one another. The encapsulation material encases or surrounds the lead frame, dice, bonding wires, etc., while electrical contacts (e.g. pins) of the lead frame remain exposed for use in connecting the packaged integrated circuit to other systems. Because of the low wetting angles produced by the oxygen plasma, voids at the interface of the encapsulation material and the circuit assembly are reduced.

An example of the parameters applied to generate the argon plasma in block 304 and the oxygen plasma in block 310 is shown in Table 1 below.

TABLE 1 Argon Plasma Oxygen Plasma RF Power 450 watts 250 watts Pressure 740 mTor 200 mTor Time  60 s  60 s Argon  92 s.c.c.m.  0 s.c.c.m. Oxygen 230 s.c.c.m.  70 s.c.c.m.

The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications. 

What is claimed is:
 1. A method for packaging an integrated circuit, comprising: applying an argon plasma to a first die, the first die including input/output pads; connecting input/output pads of the first die to terminals of a lead frame via palladium coated copper wires; applying an oxygen plasma to the first die and the palladium coated copper wires; encapsulating the first die and the palladium coated copper wires in a mold compound after application of the oxygen plasma.
 2. The method of claim 1, further comprising connecting the first die to a second die via gold bond wires prior to applying the oxygen plasma.
 3. The method of claim 2, further comprising connecting a capacitor of the first die to a capacitor of the second die via one of the gold bond wires.
 4. The method of claim 1, further comprising connecting the first die to a second die via palladium coated copper wires prior to applying the oxygen plasma.
 5. The method of claim 1, further comprising connecting input/output pads of a second die to terminals of the lead frame via palladium coated copper wires.
 6. The method of claim 5, wherein applying the oxygen plasma comprises applying the oxygen plasma to the second die and the palladium coated copper wires connecting the input/output pads of the second die to terminals of the lead frame.
 7. The method of claim 5, wherein the encapsulating comprises encapsulating the second die and the palladium coated copper wires connecting the input/output pads of the second die to terminals of the lead frame in the mold compound.
 8. The method of claim 1, further comprising radiating approximately 250 watts of radio frequency power into oxygen gas to produce the oxygen plasma. 